The really hot and humid weather during the BCG Tour ride from Kajang to Melaka and back over the past weekend got me thinking again about hydration.

Drink 2 liters / 68 fl oz (or eight glasses) of water every day.

Lose more than 2% of your body weight and your performance will decline by x%.

Words to this effect have been repeated over and over in sports, health and lifestyle magazines. They have become burnt into the minds of cyclists the world over.

It turns out that there is no scientific method behind those numbers. Exercise physiologist Stacy Sims, Ph.D., a hydration researcher at Stanford University, says that the recommendation to drink 2 liters per day don’t take into account gender, environment, altitude, and fitness level—factors that could affect fluid intake needs.
Read more at Are You Overhydrated?

Alan McCubbin, an Accredited Sports Dietitian, Accredited Practicing Dietitian and the President of Sports Dietitians Australia, points out that the recommendation that athletes drink enough fluid to prevent a loss of body weight from sweat of more than 2% during exercise is based on studies using performance tests that don’t resemble real world sporting events.
Read more at Hydration for Cyclists: How Much Do We Really Need to Drink?

It is clear that dehydration does have an effect on cyclists and other athletes. The physiological responses include:

I have lost count of how many times I have had an elevated pulse rate, an increased core temperature, and shortness of breath while on long, hot rides. I sweat more profusely than most, and so am probably losing at least 1 liter / 34 fl oz of fluid per hour of particularly hot and humid days. Perhaps more. So it is no surprise to me that I need to constantly watch my hydration levels.

Which brings me back to the question of how much I should drink while on my bicycle? I hadn’t previously considered quantifying the amount I drink in the course of a long bike ride. But a comment from a fellow participant in the BCG Tour to Melaka got me thinking. He said that one bidon (that is fancy French cyclist speak for “bottle”) lasts him between 60km and 80km / 37mi to 50mi.

I don’t know if the fluid replacement rate for a cyclist is constant over time, but let us assume that it is. Let us also assume that one bidon has a capacity of 620ml / 21fl oz. Using these assumptions, this gentleman would have drunk between 0.87 and 1.16 liters / 29 and 39 fl oz over the 112km / 70mi from Melaka to Kajang.

I drank about 5.5 liters / 186 fl oz over 112km / 70mi last Sunday. A combination of the following:

Water splashing into glass

glass of apple juice and green apples isolated on white

With a Nuun tablet dissolved in the water in each of my bidons when I started, to replace lost electrolytes.

What I do not know of course is whether this gentleman only drank from his bidon while riding, or also had other drinks during rest stops.

Whatever the case, the answer to the question is clear – or not, depending on how you look at it. The amount a cyclist needs to drink to stay adequately hydrated is a very personal thing. As pointed out by Stacy Sims, our body type – our height, weight, and a gendered predisposition to muscle or fat – will have an impact on the amount of fluid we require.

The environment at ride time is also a major determinant of how much you need to drink. I have ridden 60 to 80km on just one bidon. But that was in the cool of a Netherlands spring day. There is no way I could have survived on just two bidons in the heat of last Sunday.

So while there are guidelines, they may not apply to you as a unique individual, and to the conditions at the time you are riding.

Your starting level of hydration is likely to be important. If you start a race already partially dehydrated, then the amount you need to drink to satisfy thirst and prevent performance declines will likely be greater.

Which prompts the question of when to drink?

The Google consensus is to sip on 500 to 750 ml / 17 to 25 fl oz of isotonic (see below) carbohydrate sports fuel in the two hours before a long ride to ensure optimal hydration and fully stocked up energy reserves.

Then, during the ride, the key point to remember is not to wait until you’re thirsty to start drinking. Drink little and often right from the start, even if you don’t feel thirsty yet. If you are feeling thirsty, you are already dehydrated. Aim to take two or three good sized gulps from your bidon every ten to fifteen minutes right from the moment you roll off. Read more at Hydration on the Bike

Finally, don’t forget to continue drinking after the ride is over. Even if you drink regularly during the ride, you will still likely be dehydrated at the end. You will need to replace that lost fluid and electrolytes. I drank another 1.5 liters / 51 fl oz within an hour of finishing the BCG Tour ride back to Kajang. And more until I went to bed that night.

A final point to make is that it is possible to over-hydrate. Drinking too much can lead to hyponatremia, which is a dangerously low level of sodium in the blood. It is some consolation to know that you really have to work at it to drink too much. Most people can process about a liter or so per hour. That is 1.6 bidons per hour.

The 5.5 liters / 186 fl oz I drank last Sunday sounds like a lot. But the total ride duration was just over six hours. So I drank about a liter an hour, including a 900ml / 30 fl oz bottle of chocolate milk that I finished in one sitting.